Article having diffuser holes and method of making same

ABSTRACT

The diffusion opening in the cooling hole of an airfoil is formed by an EDM process in which the outwardly flaring sidewalls of the opening, rather then having surfaces that are approximated to be smooth by having many small ribs formed therein, are formed with a relatively few ribs with both longitudinally extending and radially extending surfaces that are substantially greater in dimension than those as normally formed. In this manner, the machining process is simplified and expedited, while at the same time, the cooling efficiency is increased.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of, and claims priority to,U.S. patent application Ser. No. 11/041,791, filed Jan. 24, 2005, titled“Article Having Diffuser Holes and Method of Making the Same”, andincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

This invention relates generally to a method for producing a hole in astructure and, more particularly, to an improved method of forming adiffuser section of an airfoil cooling hole by way of rapid electrodischarge machining (EDM).

Airfoils, such as turbine blades and vanes, are typically exposed tohigh temperatures ranging from about 800 degrees C. to 1600 degrees C.within a gas turbine engine. One method of protecting airfoils from suchextreme temperature conditions includes film cooling. Film coolingcomprises the method of passing pressurized air through cooling holes,thereby allowing the air to pass over the exterior of the airfoil as thecombustion gases encounter the airfoil. The geometric shape of thecooling holes includes both cylindrical holes and shaped holes.Cylindrical holes comprise holes generally having a circular crosssection through the entire exterior wall of the airfoil, therebyallowing the pressurized air to pass through the airfoil. Manufacturingprocesses used to manufacture cylindrical holes are discussed in U.S.patent application Ser. No. 09/356,528, which is owned by the assigneeof the present invention and hereby incorporated herein by reference.

Shaped holes, alternatively, include both a cylindrical meter sectionand a diffuser section. The cylindrical meter section allows thepressurized air to pass through the airfoil, and the diffuser sectionassists in directing the pressurized air over the airfoil's exteriorsurface. In order to direct the pressurized air as close as possible tothe exterior surface of the airfoil, the shape of the diffuser sectiondiverges outwardly from the cylindrical meter section to the airfoil'sexterior surface. A plurality of shaped holes are simultaneously formedby a “comb type” EDM electrode having a plurality of “teeth” orelectrodes that are advanced into the airfoil.

The present method for manufacturing shaped holes is conventional EDM,wherein an appropriately shaped electrode contacts a structure that istypically immersed in a dielectric fluid. Near contact between theelectrode and the structure, combined with a pulsed voltage, creates aspark between the electrode and the structure, thereby causing thestructure to erode in the shape of the electrode.

A preferred shape for a diffuser section of an airfoil cooling coil is aflared trapezoidal shape that diverges outwardly from the cylindricalmeter section. Accordingly, it has been common to use a shaped EDMelectrode which is advanced in a single stroke toward the meter sectionto obtain a shaped diffuser section with smooth, linear walls.

Although effective, the use of a relatively large shaped electrode was arelatively slow process, and production efficiencies and designlimitations required that a different method be devised. Accordingly, arapid EDM process was developed wherein the larger shaped copperelectrode was replaced with a relatively tiny solid or hollow copper orbrass tube that reciprocates back and forth across the width of thediffuser section as it advances in steps along the length of thediffuser section to form the trapezoidal shaped diffuser section.However, with this process it is impossible to obtain a trapezoidalshaped opening with a smooth linear wall and instead, the smooth wall isreplicated by the formation of staggered steps with a gradualoff-setting of the steps as the electrode is advanced toward the metersection. Thus, for a diffuser length of 0.100 inches, the electrode isadvanced about 100 times so as to form 100 sub-steps along the wall ofthe diffuser. This number may be adjusted for design purposes.

From a performance standpoint, the diffuser formed by way of the rapidEDM process is substantially equivalent to the diffuser section formedby way of the shaped electrode. In each case, the diversion angle of thediffuser section is limited to about 10 degrees. If a diffuser is formedhaving angles greater than 10. degrees, which is desirable for purposesof effective cooling, then the air flowing through the diffuser sectionwill separate from the boundary walls thereof, resulting in inefficientand inadequate cooling of the airfoil.

SUMMARY OF THE INVENTION

Briefly, in accordance with one aspect of the invention, the practice ofapproximating the smooth conical wall by the electrical dischargemachining while advancing the electrodes in minute incremental steps isabandoned and instead, the electrode is advanced in relatively fewerincremental steps across the longitudinal depth of the formed hole tothereby provide relatively longer and deeper, in the radial dimension,notches in the side wall boundaries of the formed holes. In this way,the tendency of the airflow to separate from the walls of the formedhole are lessened such that the included angle of the trapezoidal shapedhole can be increased beyond the usual 10 degrees.

By another aspect of the invention, the number of incremental steps thatthe electrode is advanced longitudinally toward the work piece isreduced from around 100 to a number in the range of 4-12, and theincluded angle of the shaped hole can be increased to 30 degrees.

By yet another aspect of the invention, for a diffuser hole formedhaving a longitudinal depth of about 0.1 inch, the longitudinal lengthof each notch formed in the side wall is in the range of 0.002 inches to0.010 inches, and the radial depth thereof is in the range of 0.000inches to 0.006 inches.

In the drawings as hereinafter described, a preferred embodiment isdepicted; however, various other modifications and alternateconstructions can be made thereto without departing from the true spiritand scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of these and objects of the invention,reference will be made to the following detailed description of theinvention which is to be read in connection with the accompanyingdrawings, where:

FIG. 1 is an elevational view of a diffuser opening formed in a portionof an airfoil in accordance with the prior art;

FIGS. 2A and 2B are schematic illustrations of a diffuser wall surfaceas formed by way of the prior art method and by way of the presentinvention, respectively;

FIGS. 3A and 3B are top view illustrations of “air solid” patterns thatrepresent a diffuser opening formed by way of the prior art method andthe present invention, respectively.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a diffuser is shown generally at 10. Thediffuser 10 could be used on any suitable article, but will be describedherein as being located on an airfoil of a gas turbine engine. Thediffuser 10 includes a cylindrical metering hole 11 and a diffuseropening 12 having the shape of a flared trapezoid in cross-section. Thatis, the cylindrical opening 11 passes through the wall of the airfoiland when it reaches the outer surface thereof, the trapezoidal shaped(in cross-section) diffuser opening 12 allows for the continued flow ofthe cooling air along the surface of the airfoil to maintain the coolingeffect.

The shape of the diffuser opening 12 is defined by the angle θ, which isthe angle between the axis C/L of the cylindrical opening 11 and theside walls of the diffuser opening 12. This angle has traditionally beenmaximized in order to maximize the area over which the cooling air isdispersed across the surface of the airfoil. However, as a practicalmatter, the angle θ has been limited to approximately 10 degrees becauseboundary layer separation typically occurs above that angle.

As will be seen in FIG. 1, a typical dimension for the diameter of thecylindrical hole 11 is 0.015 inches, and a typical longitudinal lengthof diffuser opening 12 is 0.100 inches. These dimensions may vary, ofcourse, to accommodate any particular application.

As will be seen in FIG. 2A, wherein one side 13 of the diffuser opening12 is shown for simplicity, the cylindrical opening 11 and the shapeddiffuser opening 12 will extend longitudinally along the z-axis, whilealong the x and y axes, the cylindrical opening 11 is defined by adiameter of x=y, and the diffuser opening 12 is trapezoidally shapedwith a defining angle of θ such that the x-axis dimension (i.e. in thetransverse direction) varies along the z-axis from a minimum at thetransition point 14, between the cylindrical opening 11 and thediffusion opening 12, to a maximum at the other end of the diffusionopening 12. The y-axis dimension, on the other hand varies from aminimum at the transition point 14 to a maximum at the downstream endthereof.

Diffuser openings 12 have been formed by a rapid EDM process wherein awire electrode is advanced in small steps along the z-axis, and at eachstep, it is reciprocated across the across the x-axis to erode thematerial for the purpose of forming the diffuser opening 12. Since theindividual steps or ribs 16 were very small (i.e. on the order of 1 mil,or 100 steps along the longitudinal length of 0.100 inches), a smoothsurface on the diffuser wall 13 was approximated. In this regard, it hasgenerally been understood that the cooling effect of the diffusionopening 12 would be maximized by making the steps as small as possibleand thereby approximating a smooth wall surface as close as possible.With this process, it is also understood generally, that the angle θ waslimited to 10 degrees because of boundary layer separation that wouldotherwise occur above this level.

In accordance with an embodiment of the present invention as shown inFIG. 2B, a diffusion opening 17 is formed having sides that no longerapproximate a smooth surface but rather define a relatively roughsurface. That is, as shown along the one side 18, the individual stepsor ribs 19 are relatively long (i.e. on the order of 10 ml along thez-axis, or 10 steps over a longitudinal length of 0.100 inches).

As shown in FIG. 2C, the individual steps 19 have a longitudinal lengthor “rise” of “a” along the z-axis and a transverse dimension or “run” of“b” along the x-axis as shown. The applicants have found that the numberof steps 19 is preferably maintained in the range of 4-12.

The geometry of the diffuser openings 17 has been described in terms ofa diffuser opening with a longitudinal length of 0.100 inches.Accordingly, if that length is changed, the number of steps and theirdimensions “a” and “b” would be changed accordingly. It has beendetermined that the diffuser angle θ may vary from 5 degrees to 30degrees, the rise dimension “a” may vary from 0.002 inches to 0.010inches, the run dimension “b” may vary from 0.0000 inches to 0.006inches, and the ration of rise to run, a/b, may vary from 1.7 to 12.

While the present invention has been particularly shown and describedwith reference to preferred and alternate embodiments as illustrated inthe drawings, it will be understood by one skilled in the art thevarious changes and detail may be effected therein without departingfrom the true spirit and scope of the invention as defined by theclaims.

1. A method of forming a shaped hole in a structure comprising the stepsof: providing an electrical discharge machining electrode withassociated power pulsing capability; advancing in incremental steps,said electrode along a longitudinal access toward a structure; at eachstep, moving said electrode transversely and in a reciprocating patternbetween boundaries defining a trapezoid with an included angle of θ toremove material from the structure, wherein the number of incrementalsteps in which the electrode is advanced during the entire formingprocess is limited to a range of 4 to
 12. 2. A method as set forth inclaim 1 wherein said structure is an airfoil.
 3. A method as set forthin claim 2 wherein said shaped hole fluidly communicates with acylindrical hole passing through a wall of said airfoil.
 4. A method asset forth in claim 1 wherein said included angle θ is greater than 10degrees.
 5. A method as set forth in claim 1 wherein said incrementalsteps result in said shaped hole having a plurality of ribs with thelongitudinal depth of each rib being in the range of 0.002 inches to0.010 inches.
 6. A method as set forth in claim 1 wherein said ribs havea transverse dimension in the range of 0.000 inches to 0.006 inches. 7.A method as set forth in claim 1 wherein said incremental steps resultin a shaped hole having a plurality of ribs with the ratio of theirlongitudinal depth to their transverse dimensions being in the range of1.7 to
 12. 8. A method as set forth in claim 1 wherein said electrode isof the wire type.
 9. A method of producing an electrical dischargemachined cooling hole in the diffuser section of a gas turbine enginecomponent comprising the steps of: providing a wire EDM electrode andmeans for supplying pulsed current thereto; placing said electrode inclose proximity to a surface of the component and pulsing currentthereto while moving said electrode transversely between boundariesgenerally defining a trapezoid shape to remove material from thecomponent; and advancing said electrode longitudinally in incrementalsteps toward the component and repeating said material removing step foreach incremental step, wherein the longitudinal spacing of theincremental steps is such that the total number of incremental stepsover the entire longitudinal length of the cooling hole is in the rangeof 4-12.
 10. A method as set forth in claim 9 wherein said component isan airfoil.
 11. A method as set forth in claim 10 wherein said coolinghole fluidly communicates with a cylindrical hole passing through thewall of said airfoil.
 12. A method as set forth in claim 9 wherein theboundaries of said trapezoidal shape form an included angle .theta. withthe longitudinal axes thereof, and the angle θ is greater than 10degrees.
 13. A method as set forth in claim 9 wherein the boundaries ofsaid trapezoidal shape have a plurality of ribs formed therein, and thelongitudinal depth of each of said ribs is in the range of 0.002 inchesto 0.010 inches.
 14. A method as set forth in claim 9 wherein thetransverse dimension of each of said ribs is in the range of 0.000inches to 0.006 inches.
 15. A method as set forth in claim 9 wherein theboundaries of said trapezoidal shape have a plurality of ribs formedtherein, and the ratio of the rib longitudinal depth to the ribtransverse dimension is in the range of 1.7 to
 12. 16. A method offorming a diffuser hole in an airfoil comprising the steps of: forming acylindrical hole extending from an inner end through an outer wall ofthe airfoil to an outer end; advancing an electrode longitudinallytoward said cylindrical hole outer end and pulsing current thereto toerode material; advancing said electrode laterally and continuing topulse current thereto to form a segment of a diffuser opening leadingfrom said cylindrical hole outer end to a point longitudinallydownstream thereof; continuing to advance the electrode longitudinallyin a plurality of steps toward said cylindrical hole outer end andrepeating the lateral advancement and pulsing at each step, with theextent of lateral advancement being decreased as the steps approach thecylindrical hole, wherein the total number of steps of advancement inthe longitudinal direction is in the range of 4-12.